I'm considering buying a Kindle principally for academic textbooks. The availability of content at the moment is OK, but not great (I'd say it's roughly 40% of what I'm interested in). Since this is the first time I've ever looked into this, I have no point of comparison!

Here's a thread tracking the growth of Kindle formatted books. At the moment the total is 135,583.

As far as the number of academic books, you'll have to search the Kindle Book Store to see if what you need is available.

Some publishers are beginning to Kindle-ize textbooks, so maybe a book you need may be available in the future.

It looks like they're rapidly adding back catalogue (I'm assuming books aren't released at the rate of several thousand per month!) so every reason to be hopeful. The slightly weird thing is that many of the titles I'm interest in come from a single publisher (Springer), and yet some are available whilst others are not. I've emailed them to ask about their policy, but no reply so far.